This 39-year-old patient was referred in June 2018 for the fitting of scleral contact lenses after being diagnosed with Keratoconus.
His uncorrected distance visual acuity was 6/60- OU.
Refraction June 2018:
R: -7.00 -3.50 x 77 6/12
L: -7.75 -2.00 x 119 6/9-
Although with spectacle correction, V/A was reasonable for advanced keratoconus, he still suffered from photophobia, glare and distorted vision.
He was fitted with scleral contact lenses which provided a very acceptable 6/6 visual acuity in both eyes.
On the evaluation of spherical trial lens, a cylindrical over-refraction was noted:
R: -3.00 -1.75 x 35 6/6
L: -1.50 -1.75 x 45 6/6
There can be two reasons for the Cylindrical over-refraction
- Residual astigmatism from the Intra-ocular lens
- Or, the scleral lens creates astigmatism because it isn’t aligned properly with the sclera and flexes over the cornea to make contact with the sclera where it is standing off the furthest.
The more the lens needs to flex the higher the cyl power. To differentiate between internal astigmatism and astigmatism from lens flexure, we perform corneal topography with the lens on the eye. If the topography with the lens on the eye does not reveal any significant toricity, the cylindrical component can be attributed to internal astigmatism, as was the case this patient.
If the topography with the lens on the eye does reveal toricity of similar magnitude and direction to the cylindrical component in the over-refraction, this can be attributed to the flex of the lens, and can be solved by adding a higher sagittal value to the steeper meridian of the scleral lens, giving it a better peripheral alignment with the sclera to eliminate the flexure of the lens on eye.
In this case, it was as a result of internal residual astigmatism. The lens used had a toric periphery and appropriately aligned with the scleral showing very little cyl due to lens flexure. To correct the cylindrical power, in this case, front surface cyl power had to be added at the correct axis.
It is important to get the exact axis orientation of the scleral lens, which is indicated by the 2 small lines on the lens periphery. The lines indicate the position of the flat meridian and should be stable, otherwise, when you add cylindrical power to the front surface you will make things worse if it rotates. Scleral lens axis orientation is very stable when the lens “locks into place” by adding enough peripheral toricity to create proper scleral alignment. Front surface toricity is an optical feature, and back surface toricity is a fitting feature. Both front and back toricity can be produced in the same scleral lens. The result can be excellent lens stability (from the back surface toricity alignment), and outstanding optical performance (from the front surface optical toricity).
How to determine the axis of the front surface cyl power:
1. Measure the position of the markings (in this case it was R = Axis 20, Left = 160)
2.The axis of the over-refraction was 35 in OD and 45 in OS
3.For OD it is easy 35 – 20 = 15 (Kaas’ rule = anti-clockwise subtract)
a.Subtract the orientation of the lens from the spectacle prescription
4.For OS, the lens rotated clock-wise, and even though it is measured at 160, it is the same as 20 degrees clockwise (Kaas’ rule = Clockwise add
b.Add the orientation of the lens to the spectacle prescription
Lens alignment with front surface toric is very important for the lens to remain on axis.
On the final fit, a plano over-refraction was obtained, and no further correction was necessary to get his vision to 6/6.
Final lens design:
R: BC 8.1 / 3750/3900 / 17.5mm L: BC 8.1 / 3850/4000 / 17.5mm
Power: -3.00 / -1.75 x 15 Power : -1.75 / -1.75 x 65